1. Field of the Invention
The present invention is related to improved bone cements and methods for preparing such bone cements. More particularly, the present invention is directed to improved polymethylmethacrylate bone cements for securing metal and plastic prostheses to bone and to the methods of preparing such bone cements.
2. The Prior Art
Surgical procedures to reconstruct or repair bones have become extremely common. For instance, a variety of prostheses are commercially available for use in repairing damaged hip joints or knee joints due to such diseases as osteoarthritis, rheumatoid arthritis, traumatic arthritis, avascular necrosis, and sickle cell anemia osteoporosis. In these procedures, the prosthesis is typically cemented in place by use of a polymethylmethacrylate bone cement.
Polymethylmethacrylate is a self-curing acrylic resin; it polymerizes at room temperature without any external application of heat. Polymethylmethacrylate bone cement comes to the surgeon in a kit form consisting of two components that must be mixed together to initiate polymerization.
The first component is a liquid mixture comprising monomeric methyl methacrylate together with a small amount of N, N-dimethyl-p-toluidine to induce setting of the mixed cement, and hydroquinone to inhibit self-polymerization of the monomer liquid. The second kit component is a polymer powder including polymethylmethacrylate. Frequently, the radiopaque compound barium sulfate is also included in the powder component to serve as an opacifier, and some commercial formulations include benzoyl peroxide as a catalyst to the polymerization reaction. Some commercial compositions of polymer powder also include substantial amounts of methylmethacrylate-styrene copolymer; this copolymer is believed to improve the mixing qualities of the cement.
Polymethylmethacrylate is a "luting" agent, rather than an adhesive; this cement does not produce any chemical bond with bone tissue to hold the prosthesis in place, but rather fills irregularities in the bone and hardens to form a mechanical interlock.
Because only a mechanical bond is involved, technique is extremely important. Failure to achieve a firm bond results in eventual loosening of the prosthesis; this, in turn, typically results in extreme discomfort to the patient, and a need for surgical replacement of the prosthesis.
The most often-used technique for reconstructing damaged bone tissue involves initially preparing the bone tissue by cutting and drilling the bone tissue so that it conforms to the shape of the securement portion of a prosthesis. Then, a number of shallow holes are generally drilled or cut into the surfaces of the bone tissue adjacent to the prosthesis in order to form projecting cavities into which cement will flow so as to form a strong mechanical interlock between the bone cement and the bone tissue.
The prepared bone surfaces are then thoroughly cleansed of all blood, fatty marrow tissue, bone fragments, and the like, so that the cement will conform to all of the surface irregularities of the prepared bone tissue. Finally, the two components of the unpolymerized bone cement are mixed.
The recommended manner of mixing the cement involves emptying the powdered component into a sterile mixing bowl followed by addition of the liquid component. The two components are then subjected to thorough mixing for about one minute until the commencement of polymerization is observed. Then, the cement can be loaded into a syringe while still quite fluid for injection into the prepared area. Alternatively, the cement can be kneaded for about another minute until it becomes dough-like, and then it is formed into a suitable shape for placement in the attachment site.
Once mixing of the bone cement is commenced, it is critical that the surgeon act quickly; the polymethylmethacrylate bone cement sets up extremely rapidly, and unless it is used quickly it will not flow into the irregularities and projecting cavities within the prepared bone tissue. Typically, it takes about two (2) minutes to prepare the cement for use, and the cement becomes too viscous to produce a reliable bond if it is not used within about five (5) or six (6) minutes.
Thus, the surgeon generally has only about three (3) or four (4) minutes within which to place the cement and prosthesis into the prepared reconstruction site. Even then, the bond is generally stronger if the cement and prosthesis are placed into the prepared site early within this time period rather than later. The earlier the cement is applied, the more fluid it is, and the more likely that it will flow into surface irregularities and projecting cavities. The prosthesis is then held in proper position by the surgeon for several more minutes while the cement continues to harden.
It will be appreciated that the need for haste is extremely disadvantageous. As the bone cement continues to harden, a delay of only seconds might mean the difference between success and failure. A host of occurrences, such as the need to further cleanse the prepared bone tissue, or a fluctuation in the patient's vital signs, or failure of the surgeon or his assistants to work quickly enough, can easily cause precious time to be lost. Unfortunately, the success or failure of the procedure is learned only later when the bond fails because of poor mechanical attachment to the bone tissue.
Further, the requirement for haste is not conducive to good sterile technique. Infection at the attachment site can result in damage to the restored joint, and infection can also require additional surgery to avoid further complications. Accordingly, the avoidance of infection at the site of the restoration is just as important as the obtaining of a good mechanical bond; hence, it is critical that the cement be maintained in sterile condition. Yet, the requirement to open two packages, to mix their contents, to position the cement and then the prothesis, and to do it all within just a few minutes leads to significant difficulties in maintaining good sterile technique. The result is that there is a significant likelihood of introducing infection into the surgical site.
Difficulties caused by the need for haste are not the only factors that relate to whether the prosthesis will be permanently secured. Even in those cases where a surgeon implants the prothesis within five or six minutes from commencement of mixing, there is no guarantee of success.
Because the cement comes in convenient preweighed and measured amounts, many surgeons assume that following a standard procedure will result in a cement that sets up in exactly the same manner in every case.
However, minor variations in the ratio of powder to liquid, variations in ambient temperature (taking into account such factors as nearness of surgical lights, whether the cement is kneaded with the hands, and the patient's body temperature), and variations in the size and shape of the cement mass to be placed all have important effects of the rate of polymerization. Thus, in some cases, polymerization occurs more rapidly than expected so that the cement is too viscous for reliable use even prior to five minutes from commencement of mixing.
In addition to the problems associated with conventional bone cements mentioned above, working with the various components of bone cement can be hazardous to the health of the individuals involved in its preparation and use. For instance, the liquid monomer is a highly volatile substance and is known to be irritating to the respiratory tract, eyes, and the liver. Since as much as 15% of the liquid monomer evaporates during the mixing step, the concentration of monomer vapor in the vicinity of the person doing the mixing can be high enough to cause injury. This is particularly true over prolonged periods of time due to the cumulative effects where the same person mixes batch after batch of the bone cement in operation after operation.
The liquid monomer component is also a very powerful lipid solvent, and it is known to cause contact dermatitis. Although it is recommended that those persons having direct contact with the cement wear several pairs of surgical gloves to minimize absorption into their bodies, this caution is frequently not observed. Even where it is observed, this precaution does not entirely block all absorption of the monomer into their hands, and long-term damage remains a significant possibility.
In addition to hazards to operating room personnel, the use of bone cement also frequently causes injury to the patient's bone tissue. For instance, the polymerization reaction is highly exothermic, and the temperature of the cement can climb to as much as 110.degree. C. during the polymerization process. Temperatures in the range of 70.degree. C. to 80.degree. C. are common.
One frequent source of failure of the prosthetic reconstruction is due to tissue necrosis at the bone-cement interface; a thin fibrous layer often forms at the interface between the cement and bone, and this tissue sometimes results in so much loosening of the prothesis that a second surgical operation is required.
Because of the foregoing problems, some surgeons have attempted to entirely avoid the use of bone cement by devising other mechanical attachment methods. Moreover, a great deal of effort and expense has been directed at attempts to provide such improved attachment methods. Other research has been directed to the development of improved bone cement formulations. However, these attempts have proved largely unavailing, and most surgeons continue to utilize polymethylmethacrylate in the maner described above, despite the serious problems connected with its use.
From the foregoing, it will be appreciated that it would be a significant advancement in the art of securing prostheses to bone by use of polymethylmethacrylate bone cement to provide methods and apparatus capable of extending the time available to a surgeon preparatory to placing the cement and prosthesis. It would also be a significant advancement if the hazards to operating room personnel due to the toxicity of constituents of uncured polymethylmethacrylate could be alleviated. It would also be very significant if improved sterile techniques could be provided for preparing the cement for placement, and if tissue necrosis at the bone-cement interface due to high temperatures could be reduced. It would further be a significant advancement if cement could be mixed in a manner that led to more predictable and reproducible results. Such advancements in the art are described and claimed herein.